In the transatlantic submarine optical fibre system designated TAT-9 the repeater supervisory sub-system provides monitoring facilities which aid submerged equipment fault location. It also enables switching between redundant laser transmitters and between redundant paths, in response to signals from a terminal. All functions except loopback are available with the transmission system in service so that incipient failures can be detected.
Typically the line code used in the system is 24B1P. That is, for every 24 binary data bits transmitted to line, a 25th bit is added, such that the 25-bit word has even mark parity. At the receive terminal, the parity condition can be checked to see if an error has occurred in any 25-bit word. The parity bit will indicate if an error has occurred, but not its position in the word.
The receive terminal is able to identify the position of the parity bit within the data stream by rather complex circuitry, but in the repeater a simple method has been devised which allows errors to be detected. This method is the subject matter of applicant's prior U.S. Pat. No. 4,121,195. Non return to zero (NRZ) data and clock are summed together to produce return to zero (RZ) pulses. These RZ pulses are passed through a divide-by-2 divider. For an even mark parity signal each word will generate even numbers of RZ pulses. Thus, provided there have been no errors, the output of the divider will always be in the same state at the time the parity bit has been processed. If the data bits are random, then the output for these bits will be random. Thus for 0 or 1 status at the output of the divider, there will be an average DC level with a small offset. The offset polarity will depend on the previous history. If a word with a single error is encountered, then there will be an odd number of pulses and hence the parity bit state will be reversed and so will the offset polarity. Provided no more errors occur the new offset state will be maintained. A low pass filter (LPF) is used to filter out high frequency components of the divider, and thus errors can be detected by noting the change in DC polarity at the output.
The arrangement just described can also be used to establish a communication channel as follows. If the integrity of the parity bits of the data is periodically violated at a slow rate at the terminal, then the offset at the output of the divider will follow these violations at the same slow rate. Periodic parity bit violation at the slow rate produces a tone which can be used as a carrier for supervisory commands.
In practice two different slow rates of signalling are used to provide two different tones, one for each direction of transmission.
The parity violations are affected intermittently so that the detected tone is modulated on and off at a predetermined pulse repetition frequency (PRF). Information may then be coded onto this carrier system by pulse width modulating the bursts of tone. For example:
______________________________________ Tone on 7.9 mS off 3.9 mS Tone on 3.9 mS off 7.9 mS. ______________________________________
Commands are sent to the repeaters as words which contain information on repeater to be addressed and facility required.